Well drilling is the process of drilling a borehole in the ground for extraction of natural resources, such as natural gas or petroleum. During the well drilling process, fluid from the surface, commonly referred to as “drilling mud,” is pumped into the well to maintain a desired pressure within the borehole. The desired pressure should be sufficiently high to inhibit the intrusion of unwanted materials into the borehole, but not so high as to cause the walls of the borehole to fracture.
One method for maintaining the desired pressure is through a drilling process referred to as Managed Pressure Drilling (MPD). The objectives of MPD are to ascertain the downhole pressure environment limits and to manage the annual hydraulic pressure profile accordingly. Choke valves are utilized during MPD to control the pressure.
In particular, when an underground void or fracture is encountered, the drilling mud will naturally drain from the borehole to fill the fracture. In some cases, the drilling mud draining into the fracture will cause gas originally trapped within the fracture to be forced up through the borehole, thereby causing a pressure spike or kick in the borehole. Failure to adequately control these kicks can lead to a blowout of the well.
Choke valves have been traditionally operated by pneumatic actuated systems, hydraulic actuated systems and electric valve systems driven by conventional brush motors. However, each of these systems has drawbacks, particularly in the harsh environment of well drilling.
Pneumatic actuated systems are generally easy to maintain, however, they have some limitations. In particular, pneumatic actuated valves require more shift force when in a static state than when the valve is in motion. This trait is commonly referred to as “stick slip.” To overcome stick slip, pneumatic actuated systems build up excess pressure, which can create a rapid movement once the valve is in motion. The resulting overshoot can delay or inhibit achieving a desired valve set point.
The air quality and temperature can also adversely affect the performance of pneumatic actuated systems. Poor air quality can cause the pneumatic actuated valves to operate below their peak performance and efficiency, which can lead to premature component failure. Moreover, in freezing temperatures the control valves, actuators, air lines, and controllers can seize, which can incapacitate the entire system.
Hydraulic actuated systems may be less likely to be affected by poor air quality, but can still be negatively affected by temperature. For example, temperatures below freezing can increase the viscosity, making the hydraulic fluid difficult to push through the system. At high temperatures the viscosity of the hydraulic fluid can decrease, thereby causing the hydraulic fluid to flow through smaller openings, requiring more flow and thus more power. Hydraulic actuated systems are also known to require a high degree of maintenance to ensure proper performance.
Electrical valve systems with brush motors offer some improvements over traditional fluid driven systems; however, these systems also have their limitations. In particular, the frequent cycling of power on and off to stop and start motion can cause the brushes to wear out, which can ultimately result in a system failure.
Additionally, a drawback common to pneumatic, hydraulic and conventional brush motor driven actuators is that they generally all are comprised of several components. Accordingly, these traditional systems add bulk and complexity to the well drilling system. Moreover, when the actuator fails to operate properly, a user must identify which of the components has failed, thereby increasing the amount of time that the well drilling system is down during maintenance.
Accordingly, what is needed in the industry is an improved actuator for a choke valve that overcomes the drawbacks presented by traditional pneumatic, hydraulic and conventional brush motor driven actuators. Moreover, what is needed in the industry is an actuator for a choke valve with no peripheral components, and that can be easily replaced, thereby reducing the amount of time that a given well drilling system is down for maintenance.